actions: No need to run a matrix

.github/workflows/build.yml

@@ -16,17 +16,14 @@ jobs:
 
   build:
     runs-on: windows-latest
-    strategy:
-      matrix:
-        java: [ 9, 11, 12 ]
-    name: Build (Java ${{ matrix.java }})
+    name: Build
 
     steps:
       - uses: actions/checkout@master
-      - name: Set up JDK ${{ matrix.java }}
+      - name: Set up JDK 11
         uses: actions/setup-java@master
         with:
-          java-version: ${{ matrix.java }}
+          java-version: 11
       - name: Assembling
         run: gradlew assemble --console=plain
       - name: Building
@@ -34,17 +31,14 @@ jobs:
 
   test:
     runs-on: windows-latest
-    strategy:
-      matrix:
-        java: [ 9, 11, 12 ]
-    name: Test (Java ${{ matrix.java }})
+    name: Test
 
     steps:
       - uses: actions/checkout@master
       - name: Set up JDK 11
         uses: actions/setup-java@master
         with:
-          java-version: ${{ matrix.java }}
+          java-version: 11
       - name: Assembling
         run: gradlew assemble --console=plain
       - name: Testing